It is known in the art of fiber optics that Bragg gratings (i.e., periodic or aperiodic variations in the refractive index of the optical fiber) exhibit a predetermined wavelength reflection profile. As is known, a fiber Bragg grating is the result of a photo-refractive effect. In particular, when the core of a photosensitive (e.g., germania-doped) optical fiber is exposed to ultra-violet radiation in a predetermined wavelength range, a permanent change in the refractive index is produced. The magnitude of the refractive index change is related to the intensity of the incident radiation and the time of exposure.
As is also known, a Bragg grating is impressed (or embedded or written or imprinted) into the core of an optical fiber by allowing two coherent nominally plane optical waves to interfere within the fiber core at a location along the fiber where the grating is desired. The resulting interference pattern is a standing wave which exists along the longitudinal axis of the fiber having an intensity variation which causes a corresponding spatially periodic or aperiodic variation in refractive index along the longitudinal axis of the fiber. For periodic variations, the grating has a peak reflection wavelength related to twice the spatial period (or grating spacing). The spatial periodicity or other spatial variations of the refractive index of the fiber, and the resultant reflectivity profile, are a function of the wavelength, amplitude, and/or phase of the two incident writing beams and/or their angle of intersection within the fiber.
The above described techniques are described in U.S. Pat. Nos. 4,807,950 and 4,725,110, entitled "Method for Impressing Gratings Within Fiber Optics", both to Glenn et al and U.S. Pat. No. 5,388,173, entitled "Method and Apparatus for Forming Aperiodic Gratings in Optical Fibers", to Glenn, which are hereby incorporated by reference.
It is also known that Bragg gratings embedded in the fiber may be used for parameter sensing or for wavelength filtering or tuning applications. Furthermore, a tube-encased grating (i.e., a grating in an optical fiber, where the grating and fiber are encased within and fused to at least a portion of a glass tube), may be used in numerous applications, such as is discussed in commonly-owned, co-pending U.S. patent applications, Ser. No. 09/399,404 (now abandoned), entitled "Tube-Encased Fiber Grating Pressure Sensor", Ser. No. 09/400,364 (now abandoned), entitled "Tube-Encased Fiber Grating Temperature Sensor", and Ser. No. 09/205,846 (now abandoned), entitled "Tube-Encased Compression-Tuned Fiber Grating", all filed contemporaneously herewith. In those cases, the fiber is encased in the tube by heating the tube and heating and fusing the tube to the fiber, as is described in commonly-owned copendng U.S. patent applications Ser. No. 09/399,495 (now abandoned), entitled "Tube-Encased Fiber Grating". When such a tube-encased grating is used, it may be desirable to write the grating into the optical fiber after it is encased in the tube, for ease of manufacturability, or to avoid having the grating affected by the heating and fusing process, or for other reasons.